Method for producing pig iron

ABSTRACT

The invention concerns a process for the production of pig iron with the help of blast furnaces or other suitable devices in which iron-containing materials are reduced and melted in the presence of fuels.  
     According to the invention, metallurgical coke can (be replaced) by cheaper organic exchange fuels, with the help of a coupling of a multi-step process for the gasification of the replacement fuels to reduction gas and coke with a blas furnace process to which the products of the gasification process are supplied together or separately at the height of the tuyeres or via these into a heating. The gasification process is also used for the pre-reduction of iron oxides, fine ore and ore concentrate.

[0001] The invention concerns a process for the production of pig iron with the help of fossil and secondary fuels, especially of hard and brown coal or of other organic fuels, such as refuse and sewage sludge, from iron ore or oxides which are made available in natural, pelleted form or in the form of ore dust or concentrate.

[0002] The invention can be used in the production of pig iron in blast furnaces or other installations which are suitable for a multi-stage carrying out of the process, such as e.g. the reduction of the ore to iron and the melting of the iron.

[0003] The carrying out of the process in the production of pig iron with the help of blast furnaces has, for many decades, remained substantially unchanged although a plurality of measures have become known for increasing the output by increase of the furnace dimensions and increasing of the process pressure and for the reduction of the coke consumption, e.g. by blowing in of other fuels, such as coal, heating oil, natural gas, coke oven gas, refuse, sewage sludge, ore, as well as the use of oxygen and the like. Results are given inter alia by Gudenau in the periodical “Stahl und Eisen” 117 (1997), No. 6 under the title “Experiments for the combined blowing in of coal dust and fine-grained iron-ore into the blast furnace” and at the 1st TIMS/IEHK Metallurgical Symposium 28/29 September, 1997, in Cairo under the title “Environmental and economical benefits by injecting iron containing recyclings and steel plant residues into the blast furnace” and the VDI with the handbook “Utilisation by smelting, secondary metal recovery and energy utilisation in the case of the thermal treatment of wastes” at the seminar on the 22nd and 23rd September, 1997, in Düsseldorf.

[0004] A fundamental description of the blast furnace processes is given, inter alia, in the textbook of Ost, H: Rassow, B: “Textbook of chemical technology”, 26th edition, Leipzig, Barth-Verlag, 1955.

[0005] Characteristic for the state of the art of the carrying out of the process in the blast furnace is the gasification of coke and of other organic fuels in countercurrent for the reduction of the iron ore so that the process heat requirement and the particular oxidation stage of the ore determine the gas composition and therewith the maximum possible fuel utilisation in the blast furnace which, referred to the oxidation potential of the fuel in practical operation, reaches approximately 75%. Therefore, indispensable component of the blast furnace process is the regenerative preheating of the process air which, outside of the blast furnace, uses a part of the chemical enthalpy of the blast furnace gas and thus returns into the process. Good blast furnace plant thus reach a fuel utilisation of about 83% in the case of the use of a high portion of expensive, scarce metallurgical coke as fuel.

[0006] In DE-OS 19 39 354 is described a coupling of a blast furnace or fan shaft furnace with an external reduction gas production, whereby blast furnace gas from the furnace is reacted with a gaseous, liquid or solid hydrocarbon preferably containing 1 to 13 carbon atoms in a tubular heater and/or heat exchanger with indirect heat supply. However, such an allotropic process possesses the disadvantage that the reaction temperature is limited to about 800 to maximum 950° C. Thus, only a reduction gas can be obtained which has a relatively high proportion of CO₂ and H₂O and thus possesses a small reduction potential with regard to iron oxide. As a result, the furnace must be loaded with a large amount of gas which leads to a sacrifice of capacity. Furthermore, the CO₂ contained in the reduction gas and the water vapour are reduced on the glowing coke in the blast furnace which also contributes to a reduction of the capacity of the furnace.

[0007] The relatively low reaction temperatures of up to maximum 950° C. in the case of this allotropic reduction gas production process lies, however, below the melt temperatures of inorganic components such as are normally contained in qualitatively low value fuels, such as hard and brown coals, refuse or sewage sludge. It follows from this that this process is only suitable for very pure and thus expensive fuels.

[0008] In DE-AS 24 13 558, the external production of a reduction gas takes place with the help of a plasma burner, whereby the reaction enthalpy necessary for the gasification of the fuel must be supplied by electro-energy. Thus, this process is very expensive and can only be carried out when cheap electro-energy is available. In the process, recycled blastfurnace gas which contains CO, CO₂, H₂ and H₂O and possibly carbon and/or hydrocarbon are ionised in an electric arc and brought into contact in a mixing chamber with a further part stream of blast furnace gas and carbon and/or hydrocarbon so that an endothermic post-reaction takes place. However, the process is also only suitable for very pure starting products, such as hydrocarbons or high-grade coal dust, since a slag removal is not possible not only from the combustion chamber but also from the mixing or reaction chamber of the reduction gas production.

[0009] Especially having regard to an ecologically-caused energy cost increase, for example by the introduction of an energy tax, the economy of this process is decreased and calls for other technical solutions for the reduction gas production, like the present invention.

[0010] From GB-PS 858 561, it is known to react solid fuels with air and/or oxygen to give a reduction gas, whereby the hot combustion products must be blown in at a temperature of about 1100 to 1600° C. over a separate pipe system into a blast or fan shaft furnace. The technical carrying out of this process is very complicated and expensive. Furthermore, such process procedures in which the gases must be supplied at these high temperatures to the auxiliary nozzles have not proved themselves in practice. The high gas temperatures would require the construction of an external coal gasification directly on the blast furnace which, as is known, is practically not realisable because of the constructional actualities of the blast furnace plant.

[0011] The technical task and the object of the invention are, therefore, to suggest new possibilities for the better utilisation of the fuels, as well as for the further substitution of metallurgical coke by other qualitatively lower grade fuels, preferably by brown or hard coal, but also refuse and sewage sludge, as well as coke dust obtained in the case of reduction gas production, and for the increase of capacity of the blast furnace.

[0012] The task is solved by analysis of the decisive courses of the process of the iron ore reduction and iron smelting with the help of the thermodynamic system iron-carbon-hydrogen-oxygen.

[0013] According to the invention, it is suggested to couple the blast furnace process with an external, multi-step process for the gasification of preferably powdered fossil and secondary as well as other organic fuels and/or coke obtained in the gasification process itself in which, in a first process step, the fuels are exothermally gasified with air and/or oxygen with addition of blast furnace gas from the blast furnace and/or water vapour under a pressure which is higher than the pressure in the blast furnace at the height of the blast tuyere and at a temperature between 900 and 1800° C. to give reduction gas, the quality of which corresponds approximately to that of the equilibrium gas on the phase boundary iron/wuestite in the thermo-dynamic system iron-carbon-hydrogen-oxygen and in which, by blowing in of further fuels in a second process step, which there partly react chemically endothermally with the hot reduction gas, the quality of the reduction gas is so improved that the mole ratio of the reducing gas components carbon monoxide and hydrogen to those of the oxidising components carbon dioxide and water vapour amount to at least 3:1 before it emerges from the gasification process and is blown into the blast furnace at the height of or via the tuyeres with a temperature of 300 to 1000° C.

[0014] The gasification of the fuels in the first process step preferably takes place above the melt temperature of inorganic components, whereby the impurities are then removed in the form of slag from the second process step.

[0015] The pulverised fuel not reacted in the second process step, present as coke dust in the reduction gas, is, according to the invention, blown with the reduction gas into the blast furnace or, with the help of an arrangement according to the prior art, arranged between gasification process and blast furnace, separated off from the reduction gas.

[0016] Advantageously, via the directed blowing in of coke dust from the second process step into the blast furnace, there takes place a regulation of the temperature gradients in the reduction zone of the furnace and thus the heat balance of the blast furnace controlled.

[0017] Modern processes should ensure an almost residue-free utilisation of the fuel. This is ensured in that preferably in the case of operational states which permit no additional blowing in of coke dust, according to the invention at such operational states the coke dust obtained can preferably again be returned to the gasification process, whereby, in this case, the supply of fresh fuel to the gasification process can advantageously be reduced or stopped in favour of the recycling of the coke dust obtained. The coke dust can thereby be recycled not only into the first but also into the second gasification step.

[0018] As a rule, the coke dust obtained is less active than the external fuel. Therefore, in the case of recycling into the second process step, for the ensuring of a greatest possible conversion, the less active coke dust is preferably first supplied to the hot gasification agent from the first process step into its direction of flow in order to utilise the higher temperature level for the activation of the less reactive coke dust. Only thereafter is the fresh fuel supplied to the gasification agent, already cooled by the blowing in of the coke dust, the lower temperature level of which suffices, however, for a substantially gasification of fresh fuel.

[0019] The coke dust can advantageously also be removed from the process and used somewhere else.

[0020] Furthermore, according to the invention, it is suggested to add iron ores or iron oxides in the form of fine ore or concentrate to the powdery fuels or to the coke dust before blowing into the second process step of the gasification process and, in the second process step of the gasification, to reduce these at least partly, to blow the mixture obtained of coke dust/pre-reduced iron ore/iron sponge with the reduction gas into the blast furnace or to separate this from the reduction gas leaving the second process step of the gasification and, for the avoidance of abrasiveness, to blow with the help of dense flow conveying systems, independently of the reduction gas, into the blast furnace or another known device, e.g. a smelting furnace, and there to melt.

[0021] The economic advantage of the invention results from the relief of the investment-expensive blast furnace plant and coking plant by external production of reduction gas and coke dust by gasification of fuels, which are considerably cheaper in comparison with metallurgical coke, with the help of gasification plant, the investment cost of which is more favourable and from an increase of the pig iron production of the blast furnace of up to 30% thereby possible.

EMBODIMENTAL EXAMPLE

[0022] In the following, the invention is to be described for two cases of use.

[0023] Case of Use 1

[0024] The description of the case of use 1, in which a two-stage gasification process 1 is coupled with a blast furnace 2, takes place with the help of FIG. 1.

[0025] In the first process step 4 of the gasification process 1, via the burner 3, hard coal pulverised fuel, which is introduced via the dense flow conveying system 10, is, as external fuel and as gasification agent, freed from dust in the purification plant 11 and in the compressor 12 is blown into the compression-increased blast furnace gas and oxygen from the air separation plant 14 which draws the air via the pressurised air system of the blast furnace plant 15, as well as water vapour from the steam system 13 and possibly hot air from the air heater 16 and there converted into reduction gas at a temperature of 1500° C. and a pressure of 7 bar by chemical-exothermic reaction. The ash of the fuels is thereby melted and flows from the first process step 4 through the second process step 5 into the waterbath 6, where it solidifies non-elutably.

[0026] The air removal from the pressurised air system 15 of the blast furnace plant reduces the requirements of the regenerative hot wind production in the air heater 16. With the help of the given possibilities for the mixing of the gasification agents air, oxygen, blast furnace gas and water vapour, the volume flow in the gasification process 1 and thus, in turn, the heat transport in the blast furnace process 2 can be controlled.

[0027] The reduction gas from the first process step 4 of the gasification process 1 is blown into the second process step 5 with a velocity of about 20 m/s and there also loaded with hard coal fuel dust introduced via the dense flow conveying system 10 and lances 7. The fuel dust reacts chemically endothermally in the second process step 5 with the 1500° C. hot reduction gas, whereby the temperature of the reduction gas sinks in the second process step 5 to 700° C. Residual coke is thereby obtained which is removed with the gas via the gas outlet 8 from the gasification process 1 in a cyclone 9. In the cyclone 9, the residual coke is substantially separated off from the reduction gas. The following Table shows the comparison of the gas compositions of the reaction gas after the first and the second process steps 4 and 5 of the gasification process 1. reduction gas composition after gas 1st process step 2nd process step components vol. % vol. % CO 46.41 50.29 CO₂ 14.97 7.31 H₂ 7.09 21.24 H₂O 9.14 3.63 CH₄ 0.00 0.01 N₂ 22.22 17.34 H₂S 0.13 0.15 COS 0.04 0.03 sum 100.00 100.00

[0028] Referred to the total coal fuel dust blown into the first and second process steps, 1.1 m³ of reduction gas are produced at normal temperature and pressure and 390 g residual coke per kg of hard coal fuel dust. The reduction gas and the residual coke are removed separately from the cyclone 9 and, for the avoidance of abrasion, blown separately via the or at the height of the tuyeres into the blast furnace.

[0029] Case of Use 2

[0030] In the case of the case of use 2, the gasification process 1 is operated as described in case of use 1 and illustrated in FIG. 1. To the burner 3 of the first process step 4 and to the lances 7 of the second process step 5 of the gasification process 1 are also supplied only external fuels via the dense flow conveying system 10, whereby to the fuels, which are supplied via the lances 7 of the second process step 5, are admixed fine ore and/or ore concentrate with the help of the dosing means 18.

[0031] The reduction gas obtained in the cyclone 9 is, as in the case of case of use 1, passed via the tuyeres into the blast furnace process 1 and/or, after increasing of pressure, as gasification agent passed via the burner 3 of the first process step 4 of the gasification process 1 or used somewhere else, whereas the mixture of residual coke, iron sponge and pre-reduced ore also obtained in the cyclone 9 is blown via the tuyeres or at the height of the tuyeres into the blast furnace process 1 with the help of the dense flow conveying system 17. 

1. Process for the production of pig iron with the help of fossil and secondary fuels, especially of hard or brown coals or other organic fuels, such as refuse and sewage sludge, from iron ores or other iron oxides, which are made available as in nature or worked up, e.g. as sinter, pellets, fine ore or concentrate, in a blast furnace process with external gasification of the fuels, characterised in that in the case of the gasification in the first process step, a part of the fuel is gasified chemically-exothermally with air and/or oxygen with addition of blast furnace gas from the blast furnace process and/or water vapour under a pressure which is higher than the pressure in the blast furnace at the height of the tuyeres and at a temperature between 1000 and 1800° C. to give reduction gas; in a second gasification process step, further fuel is blown into the hot reduction gas from the first process step, there partly reacts chemically-endothermally with the reduction gas and the mole ratio of the reducing gas components carbon monoxide and hydrogen to those of the oxidising components carbon dioxide and water vapour amounts to at least 3:1 and the reduction gas from the second gasification step is blown into the blast furnace at the height of the tuyeres with a temperature of 300 to 1000° C.
 2. Process for the production of pig iron according to claim 1, characterised in that the temperature gradient in the reduction zones of the blast furnace is regulated by the blowing in with the reductive gas of coke dust obtained from the second process step of the gasification process.
 3. Process for the production of pig iron according to one of claims 1 or 2, characterised in that coke dust separated from the reduction gas is recycled into the gasification process, whereby, in the gasification process, the supplying with fresh fuel is reduced in favour of the recycled coke dust and, in the case of the recycling into the second process step, for the ensuring of the greatest possible conversion, the coke dust is first supplied to the hot gasification agent from the first process step in its direction of flow for the thermal activation and only thereafter the fresh fuel is supplied to the cooled gasification agent by the blowing in of the coke dust.
 4. Process for the production of pig iron according to one of the preceding claims, characterised in that to the external, preferably powdery fuels, before the blowing into the second process step of the gasification process, are admixed iron oxide dusts, fine ore and/or iron ore concentrate; this mixture is blown into the second process step, where the iron oxide dusts, the fine ore and/or ore concentrate are partly pre-reduced to iron sponge; the solid mixture obtained, consisting of residual coke, iron sponge and pre-reduced ore, is passed with the reduction gas from the gasification process into a dust separator; the solids mixture separated from the reduction gas is blown into the blast furnace process or a melting trough with gasification taking place of the residual coke with air and/or oxygen and further reduction of the fine-grained ore to iron sponge. 